Conventional sewage plants generally contain a clarifying tank in which coarse components are removed from the waste water in a first step, an activated sludge tank and a secondary clarifying tank. Microorganisms are used in the activated sludge tank to decompose the fecal matter and other organic substances contained in the waste water. In the secondary clarifying tank, following on from the activated sludge tank, these microorganisms are normally separated from the waste water by sedimentation, and optionally recycled at least partially into the activated sludge tank. However, full separation of the microorganisms, particularly by a sedimentation process, is generally not possible in this case, so that there is a risk that the microorganisms, some of which constitute a health risk, can enter the environment with the waste water. In general, this is unacceptable. For this reason, the purified waste water coming from the activated sludge tank is treated further with filter units to reliably separate microorganisms which are present.
Besides filter units having plastic filter membranes, as are known, for example, from EP-A1-602560, WO-A-03/037489 or WO-A-04/091755, filter units having ceramic-based filters have also become widespread in recent years.
For instance, DE-A-2603505 describes a flat membrane module for separating processes in the liquid phase. The filters described therein are, however, only suitable for practical use in the applications mentioned above. For example, they do not have channels arranged inside the filter, through which the filtrate could be extracted. The consequence of this is that only one side of the filter can ever be exposed to the liquid to be filtered, and the filtrate must always be extracted via the respective opposite side of the filter.
DE-C-4329473 likewise presents pressure-stable inorganic filter membranes which, however, are very elaborate to produce. For instance, two halves in the shape of plates with semicircular indentations, which then subsequently form the internally extending filtrate channels, have to be sintered together.
On the other hand, DE-A-19807769 presents a mount for ceramic microfilters, into which a flat ceramic filter (membrane plate) can be fitted. To this end a recess, the contour of which corresponds approximately in plan view to the cross section of the microfilter fitted in it, is provided on the mount. The required liquid-tightness is achieved with the aid of rubber seals, which are placed between the walls of the recess and the microfilter. In this way, the liquid to be filtered can flow freely over a large part of the filter fitted into the mount, and the filtrate is discharged through the mount with the aid of a slight negative pressure via the channels provided inside the filter. The selected configuration of the mount, and of the filter unit formed therefrom, may, however, sometimes be disadvantageous since rubber seals sometimes cannot provide the required liquid-tightness. This may, in particular, be important when flushing out the filter unit. In this case, the flow of liquid is fed back through the filter, usually at a relatively high pressure, to clean the filter surface and the inside of the filter from contaminants present there (retentate with microorganisms, etc.).
A different approach from this is adopted in WO-A-07/128565 which describes fixing flat ceramic filters by adhesive bonding in their mounts to obtain a simple and reliable seal. Particularly in large filter units, which comprise a multiplicity of separate flat filters and corresponding mounts, the outlay associated with adhesive bonding was however considerable.
It could therefore be helpful to provide a filter system without the disadvantages mentioned above. In particular, it could be helpful to provide a filter system which has a structure that is as simple as possible and which is produced in few steps. When assembled, it should have a high degree of leaktightness.